Compression methods for audio signals have been established in the art which adhere to the traditional paradigm of perceptual audio coding by coding a spectral representation of the input signal. This approach applies coding in the frequency domain rather than in the time domain of signal. However, even for other signals, such as video signals, spectral frequency domain coding is possible.
For example, coding according to the MPEG 1- or MPEG 2-layer 3 (mp3) audio format has been established as a de-facto standard in the Internet at least as far as audio file distribution and archiving is concerned. Other frequency domain compression methods have, however, also been established as standards, such as the advanced audio coding (AAC) of MPEG-4, AC-3 of Dolby and other frequency domain encoding methods. The success of these compression methods has also created new markets for hand-held devices that are dedicated to the playback of such compressed audio files.
In depth explanations of the compression methods may be found in
K. Brandenburg, G. Stoll, “ISO-MPEG-1 audio: a generic standard for coding of high-quality digital audio”, J. Audio. Eng. Soc., Vol. 42, No. 10, October 1994, pp. 780-792.
In mobile devices, such as mobile communication devices, or mobile consumer electronic devices, the compression standard mp3 is supported as one of the possible audio formats. One example for applying the audio formats may be ringing tones. Compressed audio files may, for instance, be used as ringing tones. Since ringing tones are typically short in duration, the user might want to create a personalized ringing tone as opposed to an audio clip extracted directly from a compressed audio file. Another example, for instance, may be an audio editor application for creating personalized user content from an existing audio content database.
Within a mobile device, a database may comprise a collection of compressed audio files. However, personalization may require audio content creation tools. These may, for example, be editing tools, allowing editing the audio content. However, editing compressed files, in particular files, which have been compressed according to a frequency domain compressing method, may not be possible. Editing in the compressed domain with standard tools is not supported due to the nature of the frequency domain compressed signals. As in the compressed domain the bit stream is not a representation of the perceptual audio file in the time domain, mixing different signals is not possible without decoding.
In addition, fade-in and fade-out mechanisms are easy to implement for time domain signals. However, the computational complexity of decoding compressed audio signals is a constraint to apply fading. Both decoding and encoding would have to be implemented in case time domain fading methods were to be used. The drawback is that compressed audio bit streams, such as MPEG Audio formats, typically require a lot of computational complexity. For instance, in mobile devices, the decoding consumes much of the processing abilities, in particular as computational resources are typically limited.
Handling compressed bit streams, in particular in the frequency domain, might however be desirable. The drawback of current systems is the lack of editing possibilities in the frequency domain. The need for complete decoding the compressed data stream prior to editing increases computation time and implementation costs. There is a need for editing compressed files without the need of decompressing. For instance, mixing different signals into one single file may be desirable.
In addition, providing fading effects, such a fade-in and fade-out, might be desired even with compressed data. For instance, in mobile equipment, these editing tools for compressed audio signals are desirable.